This application claims the priority of Japanese Application No. 8-283672, filed Oct. 25, 1996, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a catalyst for removing nitrogen oxide contained in an exhaust gas from an internal combustion engine such as a vehicle engine, and to a method of efficiently removing nitrogen oxide from the exhaust gas.
The catalyst in accordance with the present invention has a high performance of removing nitrogen oxide from an exhaust gas containing oxygen burned under a lean condition as well as an exhaust gas burned under a condition of stoichiometric air-to-fuel ratio (A/F=about 14.7), has heat resistance, SOx poison resistance, and is suitable for cleaning an exhaust gas exhausted from a lean-burn engine.
In the recent trend of natural resource saving and environmental protection, there is a social need to operate a gasoline engine for a vehicle under lean-burn conditions. In connection with this trend, there has been progress to develop a catalyst (lean NOx catalyst) which can effectively remove nitrogen oxide (NOx) in an exhaust gas containing oxygen from a lean-burn engine.
In regard to exhaust gas cleaning catalysts for lean-burn engines, there is reported a catalyst supporting an alkali metal oxide and platinum on a porous supporter, for example, such catalyst is shown in Japanese Patent Application Laid-Open No. 6-31139.
Furthermore in Japanese Patent Application Laid-Open No. 8-24643, a catalyst supporting at least one kind of palladium, rhodium and platinum, and at least one of alkali metal, alkali earth metal and rare earth metal, on the porous supporter. Here, as an embodiment of the catalyst material, one catalyst contains at least one kind of palladium, rhodium and platinum, and cerium and barium, and another catalyst contains them and further supporting La, Zr, Li, K, Mg, etc. thereon.
As environmental restrictions to vehicles are growing severer, a lean NOx catalyst is required to have a higher NOx cleaning performance, and at the same time required to endure temperature changes of burned exhaust gas accompanied by various change of vehicle speed and small amounts of catalyst poisoning components (SOx, P, Pb etc.) contained in the exhaust.
In order to solve the above-mentioned technical problems, an object of the present invention is to provide a catalyst having a high NOx cleaning performance and excellent heat resistance and poison resistance to the catalyst poisoning components in the exhaust. Another object of the present invention is to provide a method for cleaning the exhaust gas by using the catalyst.
A catalyst as in the present invention supports alkali metals, titanium and noble metals on the surface of a porous supporter composed of inorganic oxide.
As the noble metal, it is preferable to contain at least one of rhodium (Rh), platinum (Pt) and palladium (Pd), and especially it is preferable to contain rhodium and at least one of platinum (Pt) and palladium (Pd).
As the alkali metal, it is preferable to contain at least one of sodium, lithium, potassium and rubidium.
The catalyst in the present invention is capable of containing one or both of a rare earth metal and magnesium other than the alkali metal, titanium and noble metal.
The rare earth metal is preferably selected from at least one among lanthanum and cerium.
Support amounts of alkali metal, titanium, noble metal, and rare earth metal are preferably to be the following weight % of metallic element to porous supporter of 100 weight %: Alkali metal is 2.5 to 27 weight %, titanium is 1 to 8 weight %, rhodium in noble metal is 0.05 to 0.3 weight %, platinum is 0.5 to 3 weight %, palladium is 0.5 to 15 weight %, and rare earth metal is 5 to 30 weight %.
Moreover, the support amount of magnesium is preferably 1 to 40% as a weight ratio of the magnesium support amount to support amounts of alkali metal and magnesium.
As the supporter, alumina, titania, silica, magnesia, mixture of silica and aluminum, and various kind of multiple oxide may be used. Especially, almina or multiple oxide of lanthanum and aluminum are desirable. It is preferable for the multiple oxide of lanthanum and aluminum to consist of lanthanum 1 to 20 mole % in mole ratio and aluminum as a remainder.
It is desirable for the porous supporter to be coated on a honeycomb substrate.
In the catalyst of the present invention, the porous supporter is supported with Ce first, Mg, Na and Ti next, with Pt and Rh, and finally with Mg again.
The catalyst in the present invention is arranged in an exhaust gas passage of the engine, and NOx which is contained in the burnt exhaust, when the air-fuel ratio is lean is reduced so as to be removed by reacting with carbon monoxide or a hydrocarbon.
The reason the catalyst in the present invention has a high purification capacity for NOx is that a high affinity for NOx is created by Ti coexisting with alkali metal NOx is adsorbed on a surface thereof, and the adsorbed NOx is reduced by a coexisting with noble metal.
NOx purification of lean exhaust deteriorates progressively while the catalyst is continuously used for a long time. This is because oxygen is in the lean exhaust and carbon monoxide or hydrocarbon as a reducer for NOx is oxidized. Accordingly, it is preferable to change into a rich condition for several seconds to several minutes, and to return to the lean condition again. Thus, the nitrogen oxide purification rate is measured and if the measured value falls below a set point, it is preferable to change. Moreover, it is preferable to measure the NOx concentration in the exhaust and to drive it in a fixed time by changing the burning condition of the engine into the theoretical air-fuel ratio or the rich condition if the measured value falls than the set point.
Methods of preparing the catalyst applicable are physical preparation methods such as an impregnation method, a kneading method, a coprecipitation method, a sol-gel method, an ion-exchange method, a vapor-deposition method and so on and chemical preparation methods utilizing chemical reaction.
Starting raw materials for catalyst preparation are various chemical compounds such as nitrides, acetates, chlorides, sulfates, carbonates, organic compounds of alkali metals, Mg, earth metals, noble metals; sol of metal oxides deflocculated by an acid or a base; metals and metal oxides; and so on. In the same way, relating to the starting raw materials for the Ti, the various chemical compounds and sol may be used. As sol of metal oxide, acid titanina such as sol of titanium nitride and sol of basic titanina may be used. In a case of using a mixed sol of an acid titanina and an acid alkali metal solution, the starting raw materials are preferable because particles of titania having poison resistance to sulfur oxides (Sox) attached with particles of alkali metal oxide on the surface are supported by a porous supporter.
The catalyst in accordance with the present invention has a high cleaning capability not only for NOx but for hydrocarbons. However, in order to attain a higher cleaning capability for hydrocarbons, it is effective to dispose a hydrocarbon burning catalyst with the catalyst of the present invention in a flow passage of the exhaust gas. In regard to combining methods, it is preferable to place the hydrocarbon burning catalyst in the rear stage of the exhaust gas cleaning catalyst. As the hydrocarbon burning catalyst, a three way catalyst or a catalyst containing Pd as the noble metal may be used.
A small amount of sulfur oxides poison catalyst active components, particularly, alkali metals. In the present invention, absorbing sulfur oxides to alkali metal is suppressed by Ti having poison resistance to sulfur oxides.
Further, it is also estimated that in a reductive environment, reduction and detachment of the absorbed sulfur oxides occur through the noble metal adjacent to the alkali metal. Therefore, even if performance of the catalyst in accordance with the present invention is temporarily deteriorated by sulfur oxides, the sulfur oxides can be removed and the catalyst can be reactivated by heating the catalyst to 400xc2x0 C. to 800xc2x0 C. under the reductive environment, that is, using the burned exhaust gas below stoichiometric air-to-fuel ratio.
The catalyst in accordance with the present invention can show an effect in treatment of exhaust gas. The diesel engine is operated under an oxygen rich condition of high air-to-fuel ratio. Since the catalyst in accordance with the present invention can show excellent activity even under an oxygen containing condition, the catalyst can clean the exhaust gas ejected from the diesel engine by effectively removing nitrogen oxides.
The catalyst in accordance with the present invention has good activity within the temperature range above 200xc2x0 C. and below 600xc2x0 C., and, particularly, has high activity within the temperature range of 250xc2x0 C. to 500xc2x0 C. Therefore, it is preferable that the temperature under which the catalyst and the exhaust gas are brought in contact with each other, that is, the so-called reaction gas temperature is set to a temperature within the above-mentioned range.